there is no overall better one. It depends on where you plan to go, what you want to do, what languages you already know and how your brain is wired.
For job-oportunities the "critical languages" [google that] and the exotic ones are the better choices, but they are so challenging that you shouldn't learn them before learning an easy one to some fluency first.
The by far easiest one (with over a million speakers) is Esperanto. Next in line for native speakers of English are Norwegian, Swedish, Danish, Dutch, Frisian, Afrikaans, French, Spanish, Portuguese, Catalan, Italian and Romanian - in no particular order
Dutch, speaking French, German, English and Esperanto